During the period of 1975-76 the major effort will be concentrated on the elucidation of the mode of replication of 2 types of phage particles: "petites" with a molecular size of the genome of 0.8, and "giant" phage, containing within its head, a molecule of DNA of some 4-5 phage equivalent units. Since petites DNA cannot possibly circularize, due to the lack of redundancy, a number of approaches will be devoted to the elucidation of the requirement for circularization as a prerequisite for the association with membrane complexes, and also the mechanism of multiplicity reactivation, which will result in the production of complete genomes. This will be followed in experiments in which interparental recombination between different density classes of petite DNA will be analyzed in CsCl gradient. Replication of giant mollecules will be scrutinized in respect to the following questions: do giant molecules replicate as such, or undergo breakdown to phage equivalent size fragments prior to replication? Do giants produce progeny strands of equivalent size under the condition when recombination is inhibited by CM. All of those experiments will involve sucrose gradient and CsCl density gradient analysis. Finally, the last group of research will be devoted to the following question: - Are giants resulting from simultaneous infection between 2 mutants, heterozygotes, i.e.- do giant molecules contain segments with different sets of alleles?